Powered by batteries, the devices produce vapor laced with varying amounts of nicotine, and frequently with added flavorings, but without the smoke and odor of traditional cigarettes.

How do they work?

A battery heats a coil that turns the nicotine solution, known as "e-liquid" or "juice," into a wispy cloud of vapor. There's no flame, no ashes. Some "cigalikes" resemble traditional cigarettes, with a colored light that looks like burning ember, while others are heavily modified or include refillable e-liquid chambers.

What's in the juice?

Most juice is a mix of propylene glycol, vegetable glycerin and liquid nicotine, plus optional flavorings. Candy flavors are popular, especially among younger users. The Food and Drug Administration, which regulates tobacco products, does not yet regulate e-cigarettes, though that is likely to change. But now there's no way to know exactly what's in the juice or how it was manufactured, shipped or stored, particularly if purchased from a foreign source. An FDA analysis of some e-cigarette products found low levels of nitrosamines, ethylene glycol and diethylene glycol — all associated with cancer and other health risks — but far less than a conventional cigarette.

So they're safer than conventional cigarettes, right?

The products are too new for possible long-term health effects — good or bad — to be known.

"E-cigarettes are very controversial," Thomas Brandon, director of the Tobacco Research and Intervention Program at Moffitt Cancer Center in Tampa, recently told the Times. "On the plus side, they may be less dangerous than cigarettes and they may help people quit or reduce smoking," which kills 5 million people worldwide annually.

"But we don't know the long-term effects of inhaling that nicotine solution, all day, every day, for years," he said. "So we don't know if they're safer than tobacco. The research hasn't been done."

What could be unsafe?

Even if the juice is free of contaminants, e-cigarettes still deliver highly addictive nicotine, which has a variety of negative health effects, and may even promote the growth of some cancers, Brandon said.

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